1. Field of the Invention
The invention relates to magnetic transducers and more particularly to heads incorporating magnetoresistive material.
2. Description of the Prior Art
Inductive magnetic heads for recording and reading information on magnetic media are not commercially applicable to many recent problems for the reasons given in the referenced Brock et al application. With reference to the embodiment disclosed in the referenced Brock et al application, it was stated that two shields may be spaced a distance s apart and either equidistant or asymmetric to a magnetoresistive element. One MR element is shown centered between two ferrite elements in an article entitled "Magnetoresistive Read/Write Head" by G. W. Brock, F. B. Shelledy and L. Viele published in the IBM TECHNICAL DISCLOSURE BULLETIN, September, 1972, pages 1206-1207. Two MR elements are placed centrally between shields in a copending application of O. Voegeli entitled "Magnetic Read Head Assembly Having Magnetoresistive Elements", Ser. No. 403,704, filed Oct. 4, 1973, now U.S. Pat. No. 3,876,618, assigned to International Business Machines Corporation, each element biasing the other. An offcenter version of the latter invention appears in an article by S. D. Cheatham and F. B. Shelledy entitled "Self-Biased, Noise-Free Magnetoresistive Head", published in the IBM TECHNICAL DISCLOSURE BULLETIN, page 1862, December, 1974.
The distortion resulting from imperfect biasing of a magnetoresistive element is unacceptable in wide band recording systems. In theory, the application of a bias field to an MR element provides a desirable linear change of resistivity in response to the signal flux. However, in practice, a quadratic term is present which will give rise to undesirable second harmonic distortion of the signal. This distortion can be made acceptably small by keeping the maximum field due to the signal small with respect to the bias field. However, this approach undesirably reduces head output. The problem is especially severe in wide band systems which experience a wide range of signal strengths. Either the head must be designed for low distortion of the long wavelength signals, or the second harmonic will be large compared to the weaker short wavelength signals. In either case, the superior signal-to-noise ratio of the MR element is lost. Experiments show that in practical heads using MR elements, the head response curves depart from the quadratic form as the MR element approaches saturation so that there is an inflection point where the second harmonic vanishes for small signals. While biasing the element at this value will minimize second harmonic distortion (imperfectly, because symmetry about the inflection point will not exist), the response to large signals will contain a substantial third harmonic component--a flattening of the peaks. This is undesirable in a wide band system. Also, operation at the inflection point implies close control of the bias field which may be difficult to achieve because of power dissipation limitations.